Climb Leap - Complete Rosa's mission "Monkey See, Monkey Do" in Sequence 7. Use this ability to make longer jumps while climbing and get hand holds that would normally be out of reach.
Disarm - Complete Mario's "Evasive Maneuvers" mission in Sequence 5. Use this ability to counter incoming attacks from weapons that are normally too heavy to counter (for example, axes).

Defeating Larger or Harder Enemies
Note: This can only be done after learning the Disarm move. Sometimes you will run into the big "Golden Knights" or the men who just fight better than normal soldiers. These opponents usually have unique weapons such as lances or big axes. They can also deflect attacks and counter more often, resulting in you being killed. To take them out more easily, simply disarm yourself. Put your weapon away by pressing Down. When they attack, press [X] to counter and disarm them. This usually works in one attempt. Quickly press press [X] again to kill the unarmed man.

Smoke Bombs
When you unlock the smoke bombs, you can use them to kill guards quicker. Equip the smoke bomb, then throw it. Once the guards are coughing, switch to your hidden blade for quicker kills.

Reuse Spears and Axes
If you lose a spear or axe inside an enemy while countering him, pick up your opponent's corpse then immediately drop it. You can now collect that weapon again and reuse it.

Santa Maria della Visitazione: Giant Squid
Go to assassin's tomb. Find the large water filled room before the room where you get one of the seals required for the Armor of Altair. This room has an obstacle course. Activate the lever before going through the course and stand on the edge of the floor and look at the water. Remain idle until a large shape swims across the water. Use the lever again after the timer resets and stand on at same location. After awhile, a giant tentacle will appear.

Fast Travel
You will start the game with only being able to go between cities or other locations via a horse. This becomes difficult once more locations are unlocked and you have to go back and forth between them frequently. You will be able to unlock and use fast travel stations in the game. These also appear on the map to make finding them easy. Using this option does have a cost of 100 Florins, but it is worth that amount to transport to your destination instead of a long horse ride.